1. Field of the Invention
The present invention relates to an optical writing device, and more particularly to an optical writing device for forming an electrostatic latent image on a photoreceptor, and an image forming apparatus comprising the optical writing device.
2. Description of Related Art
In regard to electrophotograhic image forming apparatuses such as printers and copying machines, recently, a demand for downsizing is getting stronger. In order to comply with the demand, optical writing devices of a kind that are called printer heads are being changed from an optical scanning type using a conventional laser diode as a light source to a line optical type having very small light-emitting elements, which correspond to dots, arranged in a line.
As an example of the line optical type, an optical writing device using light-emitting diodes (LEDs) as light sources has been developed. Further, recently, using organic EL elements as light sources is suggested. In a case of using organic EL elements, the light-emitting portions and a drive circuit section therefor can be mounted on a single substrate, while in a case of using LEDs, the light-emitting portions need to be mounted on a substrate separate from a drive circuit section therefor. Therefore, in terms of cost, using organic EL elements is more advantageous than using LEDs.
However, an organic EL element, in principle, has the following light-emission degradation characteristics: the quantity of emitted light becomes smaller as the cumulated light-emitting time increases; the rate of progression of light-emission degradation differs depending on luminance; and the degree of light-emission degradation differs depending on temperature.
Accordingly, when such organic EL elements having the light-emission degradation characteristics above are used as light sources, the cumulated light-emitting times of the respective organic EL elements are different depending on written images, and the respective organic EL elements have different degrees of light-emission degradation. In order to deal with this problem, it is necessary to carry out light-emission adjustment in a pixel-by-pixel manner.
Japanese Patent Laid-Open Publication No. 2010-87245 discloses a luminescent device having a light-receiving-element array and a light-emitting-element array mounted on a single substrate at a distance equal to or greater than a distance determined from the critical angle (critical-angle-determined distance Lc) from each other. In the structure, the efficiency of the light-receiving-element array in receiving total-reflected light is improved, and accurate light-quantity measurement can be carried out.
In the luminescent device disclosed by Japanese Patent Laid-Open Publication No. 2010-87245, however, the light-receiving-element array needs to be located at a distance equal to or greater than about 1.1 mm in a sub-scanning direction from the light-emitting-element array, thereby requiring a larger substrate. Considering that conventional substrates for this type of luminescent devices have a size of about 10 mm in the sub-scanning direction, the distance (about 1.1 mm), which appears small, is large enough to contribute to an area increase of the substrate. In the manufacture, as many elements as possible are mounted on a large-size mother glass at one time to reduce the manufacturing cost. However, as the area of one substrate increases, the number of substrates cut out from the mother glass is reduced, and accordingly, the production cost and the cost for material are increased.